Multi tool use

Insert entity with many to many related entities which exist in the database

I have two objects with many to many relationship:

public class Order
{
public int OrderID { get; set; }
public string OrderName { get; set; }
public virtual Collection<Product> Products { get; set; } = new Collection<Product>();
}

public class Product
{
public int ProductID { get; set; }
public string ProductName { get; set; }
public int OrderID { get; set; }
public virtual Collection<Order> Orders { get; set; } = new Collection<Order>();
}

// Mapping table
public class OrderProduct
{
public int OrderId { get; set; }
public Order Order { get; set; }
public int ProductId { get; set; }
public Product Product { get; set; }
}


I would like to add a new Order with a collection of existing Products (my API would have an input of ProductID array) into the database, so I perform like:

private void AddOrder(int productIDs)
{
Order newOrder = new Order();
newOrder.Name = "New Order";

// Here I have no clue which would be the correct way...Should I do
// Approach A(fetch each related entity - Product from database then add them into the collection of my new base entity - Order):
productIDs.ToList().Foreach(p =>
{
newOrder.Products.Add(_dbContext.Product.FindById(p))
}
);

_dbContext.Orders.Add(newOrder);
var newOrderID = _dbContext.SaveChanges();

// then fetch each product from database and add my new Order to its collection and save
productIDs.ToList().Foreach(p =>
{
var existedProductFromDb = _dbContext.Product.FindById(p)；
existedProductFromDb.Orders.Add(newOrder);
_dbContext.SaveChanges();
}
);
}


Do I really need the Mapping table between Order and Product in Entity Framework Core? Otherwise, what would be the correct way to deal with above scenario?

1 Answer
1

Your entities do not represent a many-to-many relationship using a joined table. In fact, you don't use the joining table at all.

So, start by fixing your entities:

public class Order
{
public int OrderID {get;set;}
public string OrderName {get;set;}

public ICollection<OrderProduct> Products { get; set; } = new HashSet<OrderProduct>();
}

public class Product
{
public int ProductID {get;set;}
public string ProductName {get;set;}

public ICollection<OrderProduct> Orders { get; set; } = new HashSet<OrderProduct>();
}


A new Order which contains many Products is as simple as a collection of new OrderProducts:

Order

Product

OrderProduct

private void AddOrder(int productIDs)
{
Order newOrder = new Order();
newOrder.Name = "New Order";

foreach (int productId in productIDs)
{
newOrder.Products.Add(new OrderProduct { ProductId = productId });
}

_dbContext.Orders.Add(newOrder);
_dbContext.SaveChanges();
}


Also, do notice that SaveChanges returns the number of affected rows in the database, not the Id of an inserted item.

SaveChanges

OrderId

OrderProduct

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